Control systems for adjusting the angular relationships between joints in a computer controlled machine, for example, an excavator having a boom, stick and bucket mounted from a base or cab platform, control the various articulated joints to properly move and position the bucket for digging or moving materials. Each joint, obviously must be individually controlled and coordinated with the other joints to obtain the desired result of position, orientation and movement of the bucket.
A number of different techniques have been used to control the operation of such excavator using tele-operation systems, eg. controlled by a joy stick. Examples of such control systems in U.S. Pat. No. 5,062,2264 issued Nov. 5, 1991 to Frenette et al and U.S. Pat. No. 5,062,755 issued Nov. 5, 1991 to Lawrence et al.
One technique to control an individual joint is to measure the joint angle and to feed this joint angle information to the system, and then the control drives the arm (joints) to a desired angle defined by manipulation of the control element, such as a computer or a joy stick. The system compares the desired angle (set by the computer or joy stick) with the actual angular position (measured) and defines the required change in angle. This change in angle is used to control the valve in a hydraulic system which controls the flow of hydraulic fluid in the cylinder to cause the desired repositioning of the arm (bucket).
These systems, since they are dealing with angles, and generally controlling the action of linear actuators, i.e. degree of extension of each actuator must be defined for the change in angle at the joint. Depending the angle of the joint, the amount of extension of the hydraulic actuator to obtain a given change in angle at the joint may vary from minuscule to very large which imparts very significant non-linearities to the control system.
An alternative system senses and set the desired length of the hydraulic actuator and then use a similar technique to that described above to control the length of the actuator. The actual length and desired length are compared and each actuator controlled accordingly. These systems have not reached the same degree of success as the control based on angle sensing because of the difficulty in measurement of degree of extension and the relatively high cost associated with such a system and a need to replace the sensing system when the cylinder is changed.
In hydraulic machines with buckets to handle material a problem may occur when manipulating the arm wherein changes in the angles of the arm (for example the boom and stick) result in repositioning of the bucket from a position wherein the material contained in the bucket is held therein to a position wherein the material spills from the bucket.
One form of closed loop control is shown in Japanese patent publication 59-195937 (A) published 7 Nov. 1984 to Honma. In this patent the control is based on the difference between the desired angle and the sensed angle and added gain applied to the difference, further supplemented by the desired angular velocity and then subjected to a non-linear function to provide an output signal to control the equipment--in this case for linear movement of the bucket of an excavator. The presence of a non-linearity in a control loop complicates the tuning (initial and/or ongoing) of the control gain applied.
Canadian patent 1,072,324 issued Feb. 2, 1980 to Haak et al. discloses another hydraulic control for controlling the attitude of a bucket on an excavator or the like. This system is based on control of flow between different hydraulic cylinders on the machine.